MONOMYAIRA. 117 



teeth, lateral and linear ; ligament placed in a marginal furrow, 

 and nearly interior, with one muscular impression in each valve 

 which is sublateral and elongated. 



Distinguished from the genus Mytilus by the beaks not being terminal but 

 placed a little below the apex. 



Modiola discors.— The Discordant Modiola. Plate XV. 

 fig. 17. Oblong-oval, very convex, produced and narrower at 

 the anterior end ; longitudinally striated on both sides, and 

 transversely in the middle ; outside green ; inside white or pale 

 pink, and somewhat pearly ; margin crenulated at the base ; the 

 beak oblique. One half inch long. Inhabits the British seas. 



TRIBE III. TRIDACNITES. 



Transverse, equivalve, the muscular impressions situated 

 under the centre of the lower margins, and extended on each 

 side. 



Genus 30.— HIFF OPUS.— Lamarck. 



Generic Character. — Equivalve, regular, inequilateral and 

 transverse ; lunule closed ; hinge with two compressed unequal 

 teeth ; ligament marginal, and external. 



Distinguished fronfi the Tridacna, by having the posterior slope closed, or 

 ne.irly so, and the inner margin dentated at that part. The spines wliich 

 arm the ribs are tubular, and are never arched or vaulted. 



Hippopus maculatus. — The Spotted Hippopus. Plate XV. 

 fig. 18. Transversely ovate, ventricose, with scaly ribs ; and 

 reddish purple spots ; lunule cordated, heart-shaped, and oblique ; 

 the margins very deeply crenulated. Eight inches broad. In- 

 habits the Indian seas. 



Genus 31.— TRW A Cl^ A.— Lamarck. 



Generic Character. — Shell regular, equivalve, inequilateral 

 and transverse ; a gaping space near the beaks ; hinge with two 

 compressed, unequal teeth ; ligament marginal and external. 



A bundle of tendinous fibres passes through the aperture of the posterior 

 slope, by which the animal affixes itself to rocks, where it remains sus- 

 pended. Although there is no nacreous substance in the valves themselves, 

 yet the animal sometimes produces very fine pearls, and of large size. 



Tridacna gigas.— The Giant Tridacna. Plate XV. fig. 



